Colouring matters of Australian plants. XXII. Ventilagone: crystal structure and an approach to synthesis

The crystal structure of the title compound, a naturally occurring tricyclic quinone, C16H16O5, has been found to be (1R*,3S*)-7,10- dihydroxy-1,3,8-trimethyl-3,4,6,9-tetrahydro-1H-naphtho[2,3-c]-pyran- 6,9-dione by single-crystal X-ray diffraction methods at 295(1) K and refined by least squares to a residual of 0.033 for 1143 'observed' reflections. Crystals are monoclinic, P21, a 8.346(8), b 16.79(2), c 4.911(5)Ǻ, β 101.17(8)°, Z 2. The molecule is shown to have the cis configuration. As an approach to the synthesis of the pigment the model compound 5-chloro-8-methoxy-1,3-dimethylisochroman has been prepared and the two racemic forms have been separated and characterized.